The objective of this proposed research program is to study organizational and functional features of segmental motor control. It iS composed of three distinct, yet interrelated components. The first addresses the issue of autogenetic reflex effects of muscle afferents onto gamma motoneurons. The proposed approach involves analyzing muscle afferent connections directly using intracellular recording of gamma motoneurons in conjunction with several techniques for activating single afferent types. By using a sequence of afferent activations during each recording session, it is hoped that some of the conflicting results in the literature may be resolved and a clearer picture of this neuronal circuitry emerge. The second component of this research program is designed to examine the functional interactions between motor units and muscle receptors. The approach here is tripartite seeking to: (1) examine the phenomenon of intramuscular localization of the stretch reflex using electromyographic as well as mechanical measurements; (2) study the anatomical arrangements between motor units and muscle receptors associated with peripheral "sensory-partitioning"; and (3) to test the hypothesis that motoneurons innervating a muscle receive a "weighted ensemble input" from homonymous muscle afferents, reflecting a disproportionate contribution from those receptors located within their muscle units' territory. The third component of this program is directed to motor control in a pathophysiological preparation. Experiments are proposed which seek to assess the functional significance of regenerative and plastic changes in the cat segmental motor system consequent to peripheral alterations in muscle innervation. Issues specifically addressed are: (1) whether or not the orderly patterns of motor unit recruitment are retained in reinnervated muscles and their controlling motoneuronal pools, and (2) what changes, if any, occur in the normal speed-force relations of the motor units of such muscles. These studies should provide both a measure of the functional correlates of muscle regeneration and insight into the mechanisms underlying orderly motor unit recruitment.